Method for producing lactide directly from lactic acid and a catalyst used therein

ABSTRACT

The present invention provides a method for directly producing lactide by subjecting lactic acid to a dehydration reaction in the presence of a catalyst comprising a tin compound, preferably, a tin (IV) compound, wherein lactide can be produced directly or by one step from lactic acid, without going through the step of producing or separating lactic acid oligomer. The method of the present invention has advantages of causing no loss of lactic acid, having a high conversion ratio to lactic acid and a high selectivity to optically pure lactide, and maintaining a long life time of the catalyst. Further, since lactic acid oligomer is not or hardly generated and the selectivity of meso-lactide is low, the method also has an advantage that the cost for removing or purifying this can be saved.

TECHNICAL FIELD

The present invention relates to a method for producing lactide directlyor via one step from lactic acid, and a catalyst used therein. Moreparticularly, the present invention relates to a method for producinglactide directly or via one step by subjecting lactic acid to adehydration reaction in the presence of a catalyst comprising a tincompound, and a catalyst comprising the tin compound used for the directproduction of lactide.

BACKGROUND ART

Polylactide (PLA) is a polymeric material having opticalcharacteristics, which has been used as a biodegradable or bioabsorbablemedical material for surgical suture thread, microcapsules for injectionor the like, and recently, is employed as an environmental-friendlybiodegradable material which can be used in the production of varioushigh-molecular products such as packaging materials, home appliances,office supplies, vehicle interior materials, or the like.

In order for polylactide to be used for the above usage, it is requiredto have a high optical purity (D-type or L-type optical isomer) and havehigh molecular weight. For this, lactide, which is a monomer used forproducing polylactide, should have high optical purity and chemicalpurity.

The following Reaction Chart 1 shows routes for producing lactide fromlactic acid or lactic acid ester. In case of using lactic acid as astarting material, lactide can be produced through a course ofRoute-1/Depolymerization or a course of Route-4. In case of using lacticacid ester as a starting material, lactic can be produced through acourse of Route-2 or a course of Route-3/Depolymerization.

The course of Route-1/Depolymerization is a conventional method forproducing lactide by using lactic acid as a starting material, whereinlactide is obtained via a two-step process consisting of firstlypolymerizing lactic acid under reduced pressure to produce an oligomeror prepolymer in a molecular weight of about 500˜5,000, and thendepolymerizing the produced prepolymer with the flow of inert gas undera reduced pressure (U.S. Pat. No. 5,274,073; U.S. Pat. No. 5,247,059;U.S. Pat. No. 5,274,127; U.S. Pat. No. 6,277,951). NatureWorks (acompany in USA) commercially produces L-type PLA bioplastic in a scaleof about 140 thousand tons a year by a manner consisting of producinglactide from L-type lactic acid via said two-step process, and thensubjecting the resulting lactide to a condensation polymerizationreaction in a polymerization reactor. Recently, Purac (a company inNetherlands) has also built a plant in Thailand and started a commercialproduction of lactide via the above two-step reactions.

However, in case of going through the two-step process, the productssuch as prepolymer and lactide are deteriorated when they reside for along period of time within the reactor, and thus there is a problem thatthe amount of byproducts produced such as meso-lactide increases duringthe production of L-type or D-type lactide. The two-step process ofPrepolymerization-Depolymerization of lactic acid has problems that partof the prepolymer or oligomer which is the reactant in theDepolymerization step is not decomposed to lactide and furtherpolymerizes or forms a waste material mixed with the catalyst componentincluded in the prepolymer. Also, the two-step process involves problemsthat a vacuum pump for making a high vacuum is required and the reactiondevice is complex to make the device cost high.

The course of Route-2 or Route-3/Depolymerization is a method usinglactic acid ester as a starting material. There are several referencesdisclosing a process via Route-2 for the production of lactide directlyor by one-step from lactic acid ester, or a process via Route-3 andDepolymerization for the production of lactide through prepolymer fromlactic acid (e.g. Korean Patent Application No. 10-2009-0043985,Japanese Patent Laid-Open No. 1999-036366, Japanese Patent Laid-Open No.1993-286966, and Japanese Patent Laid-Open No. 1994-031175).

Finally, the process via Route-4 relates to a method for producinglactide directly or by one step from lactic acid as a starting material.A method for the production of lactide by this process or a catalyst forthe production thereof has ever been suggested, but the suggested methodand catalyst have failed to provide satisfactory results in conversionratio and selectivity when considering the process efficiency.

U.S. Pat. No. 5,332,839 suggested a direct production of lactide by aprocess via Route-4. For example, it reports that lactide can beproduced directly from lactic acid without going through a prepolymer bygasifying lactic acid at a high temperature of at least 200° C. and thenreacting it in a fixed layer charged with a solid acid such as Al₂O₃. Incase of using a solid acid catalyst such as Al₂O₃, however, there arelimitations such that the productivity of lactic acid oligomer is high,the yield of lactide is low, carbon monoxide is generated by thedegradation of lactic acid during the reaction, and the life of thecatalyst is short.

U.S. Pat. No. 5,138,074 also suggested a direct production of lactide bya process via Route-4. For example, it discloses an example forproducing lactide directly from lactic acid in the presence of SnO.However, it does not disclose results such as conversion ratio of lacticacid, selectivity of lactide, production ratio of oligomer byproduct,and thus it is difficult to refer to the catalyst and reaction indetail.

Many methods for producing lactide by using lactic acid ester as astarting material (Route-2 and Route-3) and methods for producinglactide by a two-step process by using lactic acid as a startingmaterial (Route-1 and Depolymerization) have been developed. The methodfor producing lactide directly or by one step from lactic acid as astarting material (Route-4) has not been employed in a commercialprocess due to the absence of a suitable catalyst, although it has anadvantage that the process is simple when compared with the two-stepprocess by the Prepolymerization/Depolymerization of lactic acid(Route-1 and Depolymerization).

SUMMARY OF THE INVENTION Technical Subject

The purpose of the present invention is to develop a catalyst that canbe used in a process for producing lactide directly or by one step fromlactic acid as a starting material, without going through a step ofproducing or separating lactic acid oligomer, and a method for thedirect production of lactide using the catalyst.

Means for Achieving the Subject

The present inventors conducted researches for several years in order toimprove and simplify the process for the production of lactide fromlactic acid, and as a result, they found out that lactide can beobtained directly or by one step from lactic acid when lactic acid issubject to dehydration reaction in the presence of a catalyst comprisinga tin compound, and thus consummated the present invention.

The method for producing lactide directly or via one step from lacticacid using the catalyst according to the present invention hasadvantages of causing no loss of lactic acid, having a high conversionratio to lactic acid and a high selectivity to optically pure lactide,and maintaining a long life time of the catalyst.

Effect of the Invention

The method according to the present invention which can produce lactidedirectly or by one step from lactic acid without going through the stepof producing or separating lactic acid oligomer has advantages ofcausing no loss of lactic acid, having a high conversion ratio to lacticacid and a high selectivity to optically pure lactide, and maintaining along life time of the catalyst. Further, since lactic acid oligomer isnot or hardly formed or generated and the selectivity of meso-lactide islow, the method also has an advantage that the cost for removing them orfor purifying the product can be saved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction spectrum of tin/silicon mixed oxide(SnO₂/SiO₂) obtained from Example 1.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The first purpose of the present invention is to provide a method forproducing lactide directly from lactic acid by reacting the lactic acidin the presence of a catalyst comprising a tin compound.

The first purpose of the present invention can be specified by themethod comprising:

-   -   (Step 1) subjecting lactic acid to a dehydration reaction in a        reactor charged with catalyst comprising a tin compound, and    -   (Step 2) separating lactide obtained in the above step 1.

More particularly, the first purpose of the present invention can bespecified by the following steps:

-   -   preparing a reactor charged with catalyst;    -   supplying an aqueous lactic acid solution to the reactor;    -   subjecting lactic acid to a dehydration reaction and/or        cyclodehydration reaction in a reactor charged with the catalyst        to form lactide; and    -   separating lactide resulted from the above step from unreacted        lactic acid and water.

In the present invention, a tin compound means a catalyst comprising atin oxide, an organotin compound or tin salt. Tin salt can includefluoride, chloride, bromide, iodide, oxide, nitride or the like.

According to a preferable embodiment of the present invention, the tincompound can be selected from a tin (IV) compound or mixtures thereof,and the tin (IV) compound can include a tin (IV) oxide (SnO₂). The tinoxide can be understood to include a mixture comprising the tin oxide inthe form of a mixed oxide or complex oxide.

In the present invention, there is no particular limitation in thecontent of tin compound in the catalyst. However, the catalyst maycomprise the tin compound in an amount of at least 0.1% by weight of thecatalyst, in general, at least 1% by weight, particularly, at least 5%by weight, and preferably at least 10% by weight.

According to a preferable modification of the present invention, the tincompound in a catalyst comprising the tin compound can be selected froma tin (IV) oxide (SnO₂) or its mixed oxide. Also, a mixed oxide of thetin oxide can mean a mixed oxide of a tin (IV) oxide with an oxideselected from a group consisting of a metal other than tin or a tin (II)oxide.

According to a preferable embodiment of the present invention, thecatalyst comprising the tin compound can be a solid catalyst. In thepresent invention, a solid catalyst means a catalyst which exists insolid or solid matter state without being dissolved in the reactionsolution, and stays in an inhomogeneous state (heterogeneous state) withthe reaction solution. Liquid phase tin compound carried in a solidmetal oxide can be an example of a solid catalyst.

According to an embodiment of the present invention, the catalyst mayfurther comprise an oxide of metal selected from a group consisting ofTi, Al, Zn, Zr, V, Cr, Mn, Fe and Mo, or mixtures thereof. Thisadditional oxide of metal can be added in an amount that the mol ratioof the tin (Sn) compound to additional oxide of metal is generally1:50˜7:1, particularly 1:30˜6:1, preferably 1:10˜5:1, and morepreferably 1:4˜4:1.

According to a preferable embodiment of the present invention, the oxideof metal can be used in a form of nano particles or meso particleshaving a mean particle size of nano size or meso size, and specifically,may be used in a form of a nanoporous or mesoporous molecular sieve. Inthe present invention, mesoporous molecular sieve can mean a molecularsieve having mesopores having the size of 1˜100 nm, particularly 2˜50nm, but are not limited thereto. For example, in case the metal issilicon, it can be selected from a mesoporous silicon oxide selectedfrom a group consisting of SBA-15, MCM-41, Si-BEA, MFI and HMS, ormixtures thereof, but is not limited thereto.

The second purpose of the present invention is to provide a catalystused for the direct production of lactide from lactic acid, whichcomprises a tin compound, preferably tin (IV) compound.

Hereinafter, the method for directly producing lactide from lactic acidof the present invention is explained in detail.

The method for producing lactide directly from lactic acid in thepresent invention can proceed by a cyclic esterification to obtainlactic acid ester in the form of a stable hexagon dimer by asimultaneous or sequential dehydration of two water molecules from 2lactic acid molecules.

In the present invention, lactic acid can be generally provided to areactor or used in the reaction in the form of an aqueous solution oflactic acid. In general, lactic acid is industrially produced by afermentation method. For example, lactic acid can be obtained in theform of calcium salt or ammonium salt of lactic acid throughfermentation in an aqueous solution of monosaccharide such as glucoseobtained by saccharification of carbohydrate material consisting ofpolysaccharide or cellulose, etc. or in the form of an aqueous lacticacid solution through acid fermentation. Then, a purified aqueoussolution of lactic acid can be prepared via several purification andconcentration steps. There is an advantage that the volume of thereactor can be minimized as the concentration of lactic acid in theaqueous lactic acid solution increases. However, if the concentration oflactic acid gets too high, the viscosity increases, and in this case,there is difficulty in injecting to the reactor by using a liquid pump.Also, in case the concentration of lactic acid is higher than 90%, theproduction ratio of lactic acid oligomer may increase during theproduction of lactide. Thus, the concentration of lactic acid in anaqueous solution of lactic acid should be 50˜90%, preferably 60˜85%.

In the present invention, L-type lactic acid, D-type lactic acid orL-type/D-type mixed lactic acid can all be used as lactic acid.

Next, an aqueous lactic acid solution prepared as above is introducedinto a reactor charged with a catalyst and is converted to lactide by adehydration and/or cyclization reaction. There is no particularlimitation in the supply rate of aqueous lactic acid solution, but itcan be supplied at a rate of 0.1˜3 kg/h per 1 kg of catalyst,particularly at a rate of 0.5˜2 kg/h. However, if the supply rate oflactic acid is too high, the conversion ratio of lactic acid decreases,and there is a problem that the costs for separating after the reactionwould increase.

When supplying an aqueous lactic acid solution into the reactor, inertgas such as nitrogen, argon, helium or the like can be introducedtogether with the aqueous solution of lactic acid continuously orintermittently in order to supply the reactant smoothly and/or removethe product quickly. Inert gas facilitates lactide, unreacted lacticacid and any byproduct including water which are resulted after thereaction of lactic acid to be quickly removed from the reactor, andthus, it may be helpful in preventing inactivation of the catalyst andmaintaining the life time of the catalyst. The amount of inert gassupplied is 10˜300 L/min, particularly 15˜250 L/min and preferably20˜200 L/min with respect to 1 kg/h of lactic acid supplied. As theamount of inert gas supplied increases, there are advantages in thesupply of the reactant of an aqueous lactic acid solution as well as theremove of lactide, unreacted lactic acid and byproducts including waterwhich are produced during the reaction. However, there is a disadvantagethat when the amount exceeds 300 L/min with respect to the suppliedamount of 1 kg/h of lactic acid, the cost for introducing gas wouldincrease.

Meanwhile, in order to supply aqueous solution of lactic acid smoothly,a reactor installed with a dispersion plate at the inlet can be used,and in this case, the amount of inert gas required to be injectedtogether with aqueous solution of lactic acid can be reduced.

In the present invention, with regard to reaction temperature,particularly, the dehydration reaction of aqueous solution of lacticacid can be carried out at a temperature of 170˜250 (280° C.,particularly, 180˜240 (250° C. In this temperature range, the conversionratio from lactic acid to lactide is fast, and simultaneously theproduction of oligomer by the polymerization reaction of lactic acid canbe suppressed.

There is no particular limitation in reaction pressure in the presentinvention. The process of the present invention is characterized byproducing lactide even at a normal pressure. This has a great advantagein operating the process when compared to the process by theconventional process of pre-polymerization/depolymerization of lacticacid which requires high vacuum when producing lactide. However, it ispossible to carry out the reaction of the present invention even atvacuum condition for the purpose of easily collecting lactide producedafter the reaction and unreacted lactic acid. The normal pressure of thepresent invention means the atmospheric pressure or a pressure of about1 atm, but 0.8˜1.2 atm can also be deemed as normal pressure.

There is no particular limitation in the form of catalyst in the presentinvention, and the catalyst may have any form that can be charged in areactor, for example, granule, pellet, injection mold, monolith,honeycomb, thin film or the like, and a proper form can be selecteddepending on the form of the reactor.

The inventors found that a catalyst comprising a tin compound,preferably a tin (IV) compound, more preferably a tin (IV) oxide (SnO₂)is very effective in a reaction directly converting lactic acid tolactide. Further, the inventors found that in case of using the tin (IV)compound together with an oxide of an additional metal selected from agroup consisting of Si, Ti, Al, Zn, Zr, V, Cr, Mn, Fe and Mo, thecatalyst presented higher conversion ratio of lactic acid and moreexcellent selectivity of lactide than the case of using a catalystcomprising a tin compound alone. With regard to a catalyst comprisingthe tin (IV) compound together with the oxide of metal, the mol ratio ofthe tin compound to the oxide of metal can be selected from 1:50˜7:1,and preferably from 1:4˜4:1.

With regard to a catalyst comprising tin (IV) compound together with anoxide of metal, the tin compound may be present in a form carried on ormixed with the additional oxide of metal. For example, the tin compoundmay be present in the form of a mixed oxide of tin (IV) oxide and theadditional oxide of metal.

In the present invention, a preferable example of a catalyst comprisinga tin (IV) compound and an oxide of metal can include a mixed oxide oftin and silicon (SnO₂/SiO₂) or a complex oxide (Sn_(a)Si_(b)O₄)(wherein, a and b may be selected from an integer of 0 to 2, and a+b=2).Here, the mol ratio of the tin oxide to silicon oxide is 1:50˜7:1,preferably 1:4˜4:1.

According to an embodiment of the present invention, a catalystcomprising tin oxide and silicon oxide in the form of a mixed oxide(i.e., catalyst comprising tin/silicon mixed oxide) can be prepared byhaving a tin oxide precursor carried on in silicon oxide or precursorparticles thereof. Specifically, the catalyst according to the presentinvention can be produced by adding a tin compound such as tin chloride(SnCl₄) in a beaker containing nano size silica sol and stirring it,separating the resulting precipitates with a filter, drying at about100° C., and plasticizing at about 450° C. There is no particularlimitation in the drying temperature and plasticizing temperaturementioned in the above.

According to the present invention, when producing a catalyst comprisinga tin/silicon mixed oxide, it would be advantageous to have theconcentration of tin in a reaction solution for producing the catalysthigh. Also, preferably, a precipitation method using silica sol of nanosize can be used for uniform dispersion of tin. Of course, with regardto the solid catalyst used in the present invention, there is noparticular limitation in the particle size of the component in thepresent invention. In case of using the above method for producingcatalyst, a tin-silicon oxide catalyst of high concentration whose molratio of the tin to metal is at least 1:1 can be produced without greatloss of BET surface area.

An advantage of the present invention is that since the selectivity ofmeso-lactide is very low, generally 3% or less, preferably 2.5% or less,and specially 2% or less, the necessity for an additional step ofpurifying the finally resulted product of optically pure D-lactide orL-lactide or for an additional step of removing meso-lactide is reducedgreatly. In prior art, the selectivity of meso-lactide may generallyreach 10% or above, and thus a purification process or removal processmay be essential.

An advantage of the present invention is that lactic acid oligomer isnot or hardly produced, and thus the step of removing or separatinglactic acid oligomer is not essential in subsequent processes. Thus, thenecessity for a lactic acid oligomer separation column is reduced. Inprior art, in particular, according to the technology for producinglactide by pre-polymerization/depolymerization of lactic acid (forexample, a course via Route-1), lactic acid oligomer having a highboiling point is together produced as a byproduct in addition to theproduct of lactide. Thus, there is a problem that a separation columnfor removing lactic acid oligomer must be installed separately in thesubsequent processes in addition to the lactic acid/lactide separationcolumn.

A further advantage of the present invention is that lactic acidoligomer is hardly produced even when performing the dehydrationreaction of lactic acid at a high temperature of at least 200° C.

The reaction mixture obtained after the dehydration and/or cyclizationreaction of lactic acid according to the present invention comprises notonly lactide, but also unreacted lactic acid, water and a small amountof oligomer byproducts. Thus, it may go through a step of separatinglactide therefrom.

Lactide can be separated by methods such as distillation,recrystallization or solvent extraction, etc. of lactide used in theprior art. Also, a reaction distillation technique combining the step ofgoing through a direct catalyst conversion reaction of lactide fromlactic acid and the step of separating the produced lactide from othercompounds can be applied.

Also, according to the technology for producing lactide bypre-polymerization/depolymerization of lactic acid (for example, aprocess via Route-1) in prior art, meso-lactide is generated in morethan a little ratio. Thus, it is necessary to remove meso-lactide orpurify optically pure L- or D-lactide. However, according to thetechnology of direct conversion by using the catalyst according to thepresent invention, the production ratio of meso-lactide is very low, andthus the cost for removing or purifying can be saved.

According to the present invention, lactide is produced from lactic aciddirectly or by one step without forming or separating lactic acidoligomer. Thus, it has advantages of causing no loss of lactic acid,having a high conversion ratio to lactic acid and a high selectivity tooptically pure lactide, and maintaining a long life time of thecatalyst, as well as has effects of simplifying the production process,saving the processing cost and increasing production efficiency.

Further, according to the present invention, a separate apparatus anddevice for removing lactic acid oligomer is not required because lacticacid oligomer is not generated or hardly generated. In addition, sincethe selectivity of meso-lactide is lower than prior art, the cost forremoving this can be saved as well.

The present invention is not limited to the examples described below,and it is obvious to a person having ordinary skill in the art thatvarious corrections and modifications can be made as far as they do notdeviate from the idea and scope of the present invention. Thus, examplesof such modification or corrections fall within the scope of the presentinvention.

EXAMPLES Example 1 Production of Lactide by Using SnO₂(80)/SiO₂ Catalyst

37 g of SnCl₄.5H₂O and 13.75 g of silica sol (Ludox, SM30) areintroduced into a beaker and stirred, and while maintaining thetemperature at 5° C., the pH is adjusted to pH 8 with 0.2 M NaOH. Thetemperature of the resulting mixture is raised to 70° C. and the mixtureis additionally stirred for 4 hours. The resulting precipitate isseparated with a filter, dried for 5 hours at 100° C., and finallycalcinated at 450° C. for 2 hours to obtain a catalyst comprising 80% byweight of tin oxide and 20% by weight of silica.

The above obtained catalyst sample is analyzed by the X-ray diffractionanalysis to shows that peaks corresponding to tin (IV) compound (SnO₂)were mainly observed as peaks relating to tin compound confirmed fromthe X-ray diffraction spectrum (see FIG. 1). Thus, the catalystsynthesized by the above synthesis method is abbreviated as SnO₂ (80% byweight; hereinafter, unit is omitted)/SiO₂ catalyst.

A fixed layer reactor is charged with 1 g of the SnO₂(80)/SiO₂ (20-40mesh) catalyst produced in the above, maintained at 180° C. under anormal pressure, and provided with 75% L-aqueous solution of lactic acid(Aldrich) at a supply rate of 0.5 g/h simultaneously with nitrogen at asupply rate of 100 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 88%, the selectivity of L-lactideis 98%, and the selectivity of meso-lactide is 2%, and oligomer oflactic acid are detected on the chromatogram. The above results areshown in Table 1 below.

Example 2 Production of Lactide by Using SnO₂(80)/SiO₂ Catalyst

A fixed layer reactor is charged with 1 g of SnO₂(80)/SiO₂ catalyst inthe same method as Example 1, maintained at 180° C. under a normalpressure, and furnished with 75% L-aqueous solution of lactic acid at asupply rate of 1.0 g/h simultaneously with nitrogen at a supply rate of200 ml/min.

Under the above condition, the (dehydration) reaction is continued for200 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 84%, the selectivity of L-lactideis 98%, the selectivity of meso-lactide is 2%, and the selectivity oflactic acid oligomer is 1% or less. The above results are shown in Table1 below.

Example 3 Production of Lactide by Using SnO₂(80)/SiO₂ Catalyst

A fixed layer reactor is charged with 1 g of SnO₂(80)/SiO₂ catalyst inthe same method as Example 1, maintained the reactor at 180° C. under anormal pressure, and furnished with 75% L-aqueous solution of lacticacid at a supply rate of 1.5 g/h simultaneously together with nitrogenat a supply rate of 250 ml/min.

Under the above condition, the (dehydration) reaction is continued for500 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 81%, the selectivity of L-lactideis 98%, and the selectivity of meso-lactide is 2%. No lactic acidoligomer is observed in the chromatography. The results are shown inTable 1 below.

Example 4 Production of Lactide by Using SnO₂(80)/SiO₂ Catalyst

A fixed layer reactor is charged with 1 g of SnO₂(80)/SiO₂ catalyst inthe same method as Example 1, maintained at 180° C. under a normalpressure, and furnished with 75% L-aqueous solution of lactic acid at asupply rate of 2.0 g/h simultaneously together with nitrogen at a supplyrate of 300 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 78%, the selectivity of L-lactideis 98%, the selectivity of meso-lactide is 2%, and the selectivity oflactic acid oligomer is 1%. The results are shown in Table 1 below.

Example 5 Production of Lactide by Using SnO₂(80)/SiO₂ Catalyst

A fixed layer reactor is charged with 1 g of SnO₂(80)/SiO₂ catalyst inthe same method as Example 1, maintained at 180° C. under a normalpressure, and furnished with 75% L-aqueous solution of lactic acid isintroduced at a supply rate of 1.5 g/h simultaneously together withnitrogen at a supply rate of 300 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 84%, the selectivity of L-lactideis 97%, the selectivity of meso-lactide is 2%, and the selectivity oflactic acid oligomer is 1%. The results are shown in Table 1 below.

Example 6 Production of Lactide by Using SnO₂(80)/SiO₂ Catalyst

A fixed layer reactor is charged with 1 g of SnO₂(80)/SiO₂ catalyst inthe same method as Example 1, maintained at 240° C. under a normalpressure, and furnished with 75% L-aqueous solution of lactic acid at asupply rate of 1.5 g/h simultaneously together with nitrogen at a supplyrate of 300 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 96%, the selectivity of L-lactideis 97%, the selectivity of meso-lactide is 2%, and the selectivity oflactic acid oligomer is 1%. The results are shown in Table 1 below.

Example 7 Production of Lactide by Using SnO₂(40)/SiO₂ Catalyst

13.5 g of SnCl₄.5H₂O and 30 g of silica sol (Ludox, SM30) are introducedinto a beaker and stirred, and while maintaining the temperature at 5°C., the pH is adjusted to pH 8 by 0.2 M NaOH. The temperature of theresulting mixture is raised to 70° C. and the mixture is additionallystirred for 4 hours. The resulting precipitate is separated with afilter, dried for 5 hours at 100° C., and calcinated for 2 hours at 450°C. to obtain a catalyst SnO₂(40)/SiO₂ comprising 40% by weight of tinoxide and 60% by weight of silica.

A fixed layer reactor is charged with 1 g of the SnO₂(40)/SiO₂ (20-40mesh) catalyst produced in the above, maintained at 180° C. under anormal pressure, and furnished with 75% L-aqueous solution of lacticacid (Aldrich) at a supply rate of 1.5 g/h simultaneously together withnitrogen at a supply rate of 200 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 51%, the selectivity of L-lactideis 98%, and the selectivity of meso-lactide is 2%. The selectivity oflactic acid oligomer is 0%, that is, oligomer of lactic acid is notdetected on the chromatogram. The above results are shown in Table 1below.

Example 8 Production of Lactide by Using SnO₂(80)/TiO₂ Catalyst

A fixed layer reactor is charged with 1 g of SnO₂(80)/TiO₂ catalyst inthe same method as Example 1, maintained at 240° C. under a normalpressure, and furnished with 75% L-aqueous solution of lactic acid at asupply rate of 1.5 g/h simultaneously together with nitrogen at a supplyrate of 250 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 72%, the selectivity of L-lactideis 92%, and the selectivity of meso-lactide is 2%. The above results areshown in Table 1 below.

Example 9 Production of Lactide by Using SnO₂(80)/Al₂O₃ Catalyst

A fixed layer reactor is charged with 1 g of SnO₂(80)/Al₂O₃ catalyst inthe same method as Example 1, maintained at 180° C. under a normalpressure, and furnished with 75% L-aqueous solution of lactic acid at asupply rate of 1.5 g/h simultaneously together with nitrogen at a supplyrate of 250 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 66%, the selectivity of L-lactideis 89%, and the selectivity of meso-lactide is 2%. The above results areshown in Table 1 below.

Example 10 Production of Lactide by Using SnO₂(80)/ZrO₂ Catalyst

A fixed layer reactor is charged with 1 g of SnO₂(80)/ZrO₂ catalyst inthe same method as Example 1, maintained at 180° C. under a normalpressure, and furnished with 75% L-aqueous solution of lactic acid at asupply rate of 1.5 g/h simultaneously together with nitrogen at a supplyrate of 250 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 66%, the selectivity of L-lactideis 90%, and the selectivity of meso-lactide is 2%. The above results areshown in Table 1 below.

Example 11 Production of Lactide by Using SnO₂(100) Catalyst

37 g of SnCl₄.5H₂O is introduced into a beaker and stirred, and whilemaintaining the temperature at 5° C., the pH is adjusted to pH 8 with0.2 M NaOH. The temperature of the resulting mixture is raised to 70° C.and the mixture is additionally stirred for 4 hours. The resultingprecipitate is separated with a filter, dried for 5 hours at 100° C.,and calcinated for 2 hours at 450° C. to obtain a catalyst SnO₂(100).

A fixed layer reactor is charged with 1 g of the SnO₂(100) (20-40 mesh)catalyst produced in the above, maintained at 180° C. under a normalpressure, and furnished with 75% L-aqueous solution of lactic acid at asupply rate of 1.5 g/h simultaneously together with nitrogen at a supplyrate of 200 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 48%, the selectivity of L-lactideis 95%, the selectivity of meso-lactide is 3%, and the selectivity oflactic acid oligomer is 2%. The above results are shown in Table 1below.

Example 12 Production of Lactide by Using SnO₂(80)/SBA-15 Catalyst

A fixed layer reactor is charged with 1 g of SnO₂(80)/SBA-15 catalyst inthe same method as Example 1, maintained at 240° C. under a normalpressure, and furnished with 75% L-aqueous solution of lactic acid at asupply rate of 1.5 g/h simultaneously together with nitrogen at a supplyrate of 250 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 96%, the selectivity of L-lactideis 98%, and the selectivity of meso-lactide is 2%. The above results areshown in Table 1 below.

Comparative Example 1 Production of Lactide by Using SiO₂

35 g of silica sol (Ludox, SM30) is introduced into a beaker andstirred, and while maintaining the temperature at 5° C., the pH isadjusted to pH 8 with 0.2 M NaOH. The temperature of the resultingmixture is raised to 70° C. and the mixture is additionally stirred for4 hours. The resulting precipitate is separated with a filter, dried for5 hours at 100° C., and calcinated for 2 hours at 450° C. to obtain acatalyst SiO₂.

A fixed layer reactor is charged with 1 g of the SiO₂ (20-40 mesh)catalyst produced in the above, maintained at 180° C. under a normalpressure, and furnished with 75% L-aqueous solution of lactic acid at asupply rate of 1.5 g/h simultaneously together with nitrogen at a supplyrate of 200 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 16%, the selectivity of L-lactideis 95%, the selectivity of meso-lactide is 2%, and the selectivity oflactic acid oligomer is 3%. The results are shown in Table 1 below.

Comparative Example 2 Production of Lactide by Using Al₂O₃

A fixed layer reactor is charged with 1 g of gamma-Al₂O₃ (20-40 mesh) asa catalyst, maintained at 180° C. under a normal pressure, and furnishedwith 75% L-aqueous solution of lactic acid at a supply rate of 1.0 g/hsimultaneously together with nitrogen at a supply rate of 200 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 29%, the selectivity of L-lactideis 95%, the selectivity of meso-lactide is 2%, and the selectivity oflactic acid oligomer is 3%. The results are shown in Table 1 below.

Comparative Example 3 Production of Lactide by Using SiO₂(87)/Al₂O₃(13)

A fixed layer reactor is charged with 1 g of SiO₂(87)/Al₂O₃(13) (20-40mesh) as a catalyst, maintained at 180° C. under a normal pressure, andfurnished with 75% L-aqueous solution of lactic acid at a supply rate of1.0 g/h simultaneously together with nitrogen at a supply rate of 200ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 35%, the selectivity of L-lactideis 94%, the selectivity of meso-lactide is 3%, and the selectivity oflactic acid oligomer is 3%. The results are shown in Table 1 below.

Comparative Example 4 Production of Lactide by Using SnO

A fixed layer reactor is charged with 1 g of SnO (20-40 mesh) as acatalyst, maintained at 180° C. under a normal pressure, and furnishedwith 75% L-aqueous solution of lactic acid at a supply rate of 1.0 g/hsimultaneously together with nitrogen at a supply rate of 200 ml/min.

Under the above condition, the (dehydration) reaction is continued for100 hours. Gas chromatography analysis of the product showed that theconversion ratio of L-lactic acid is 55%, the selectivity of L-lactideis 82%, the selectivity of meso-lactide is 3%, and the selectivity oflactic acid oligomer is 15%. The results are shown in Table 1 below.

TABLE 1 Conversion Selectivity of Selectivity of ratio of lacticSelectivity of meso-lactide lactic acid Example acid (%) L-lactide (%)(%) oligomer (%) Example 1 88 98 2 0 Example 2 84 98 2 1 Example 3 81 982 0 Example 4 78 98 2 1 Example 5 84 97 2 1 Example 6 96 97 2 1 Example7 51 98 2 0 Example 8 72 92 2 6 Example 9 66 89 2 9 Example 10 66 90 2 8Example 11 48 95 3 2 Example 12 96 98 2 0 Comparative 16 95 2 3 example1 Comparative 29 95 2 3 example 2 Comparative 35 94 3 3 example 3Comparative 55 82 3 15 example 4

Referring to table 1, it can be understood that the conversion ratio oflactic acid is remarkably low, 40% or below, and the selectivity oflactic acid oligomer is relatively high in case of using SiO₂, Al₂O₃ andSiO₂/Al₂O₃ as catalyst as in comparative examples 1˜1 In case theconversion ratio of lactic acid is low, there is a problem that the costfor separating the lactide product from lactic acid after the reactionis very high. Considering that the lactic acid material takes a largepart in the method for producing lactide, it can be understood thatthere are limitations in using the catalyst used in comparative examples1˜3 in the method for directly producing lactide from lactic acid.

Meanwhile, as can be seen from comparative example 4, in case of usingonly tin (II) compound (SnO) as a catalyst, it can be understood thatthe selectivity of lactic acid oligomer, which is a byproduct, is highand the optical selectivity of lactide is very low, when compared withthe case of using tin (IV) compound (SnO₂).

Experimental Example 1 Continuous Production of Lactide by UsingSnO₂(80)/SiO₂ Catalyst

A fixed layer reactor is charged with 1 g of SnO₂(80)/SiO₂ catalyst inthe same method as Example 1, and then 75% L-aqueous solution of lacticacid is introduced to the reactor simultaneously with nitrogen. Thetemperature of the reactor charged with catalyst is maintained at 240°C. and the pressure of the reactor is maintained at normal pressure. Thereaction is performed for 3,000 hours at the above condition, whilecontrolling the supply speed of lactic acid and the amount of nitrogen.

Gas chromatography analysis of the product showed that the conversionratio of L-lactic acid is at least 96%, the selectivity of L-lactide isat least 97%, the selectivity of meso-lactide is 2% or less, and theselectivity of the lactic acid oligomer is 1% or less.

Experimental example 1 shows that the catalyst according to the presentinvention can maintain excellent catalyst performance for at least 3,000hours even at very high reaction temperature of 240° C.

What is claimed is:
 1. A method for producing lactide directly from lactic acid by reacting the lactic acid in the presence of a solid catalyst comprising a tin compound, wherein said tin component is SnO₂, or SnO₂ and a metal oxide, wherein the metal is selected from the group consisting of Si, Ti, Al, Zn, and mixtures thereof.
 2. The method according to claim 1, wherein said metal oxide is contained in an amount that the molar ratio of the tin compound to said metal oxide is selected from 1:50 to 7:1.
 3. The method according to claim 1, wherein said tin compound is carried or mixed with on said metal oxide, in the case that the tin component is SnO₂ and a metal oxide.
 4. The method according to claim 1, wherein said oxide is a mesoporous silicon oxide selected from the group consisting of SBA-15, MCM-41, Si-BEA, MFI and HMS, or mixtures thereof.
 5. The method according to claim 1, comprising: (Step 1) subjecting lactic acid to a dehydration reaction in a reactor charged with said solid catalyst comprising a tin compound, and (Step 2) separating lactide obtained in the above step
 1. 